High hardness materials are widely used as coatings on various types of mechanical components and cutting tools. Such coatings impart erosion and abrasion wear resistance and thus increase the erosive and abrasive wear life of objects that have been coated. The high hardness materials can also be used to produce free standing objects which are erosive and abrasive wear resistant.
Chemical vapor deposition processes can be used to produce highly erosive and abrasive wear resistant hard coatings and free standing objects. In a typical chemical vapor deposition (CVD) process the substrate to be coated is heated in a suitable chamber and then a gaseous reactant mixture is introduced into the chamber. The gaseous reactant mixture reacts at the surface of the substrate to form a coherent and adherent layer of the desired coating. By varying the gaseous reactant mixture and the CVD process parameters, various types of deposited coatings can be produced.
In U.S. Pat. No. 4,855,188, filed Feb. 8, 1988, a composite coating system comprising an intermediate layer of tungsten and an outer layer of tungsten/tungsten carbide was disclosed. The disclosed tungsten/tungsten carbide coatings exhibit an extremely hard, fine grained, non-columnar, substantially lamellar or layered structure produced by chemical vapor deposition.
The tungsten/tungsten carbide coatings disclosed in U.S. Pat. No. 4,855,188 are comprised primarily of a mixture of a substantially pure tungsten phase and at least one carbide phase wherein the carbide phase consists of W.sub.2 C or W.sub.3 C or a mixture of W.sub.2 C and W.sub.3 C. The coating system also employs a thickness ratio of tungsten to tungsten/tungsten carbide layer of .gtoreq.0.3 to get optimum erosion and abrasion wear performance. The intermediate tungsten layer as claimed is believed to improve the impact and load bearing capacity of the composite coating by absorbing impact energy and by deflecting cracks produced in the tungsten/tungsten carbide coating on impact.
It has been found that the tungsten/tungsten carbide coatings such as those described in the aforementioned U.S. Patent when deposited upon certain types of substrates, exhibit a very fine micro-crack system throughout the deposit. On many types of substrates and under many types of erosive and abrasive wear conditions, preferential attack occurs at the cracks, resulting in poor erosion and abrasion wear resistance for such coatings. Additionally, under many types of erosion and abrasion wear conditions, these coatings fail by spalling due to poor load and impact bearing capacity.
To improve the overall erosion and abrasion wear properties and eliminate cracks in the composite coatings, the aforementioned Patent disclosed limiting the thickness of the tungsten/tungsten carbide top coat to .ltoreq.14 microns (.mu.m) and using a ratio of thicknesses of tungsten interlayer to tungsten/tungsten carbide top coat of .gtoreq.0.3. Limiting the tungsten/tungsten carbide coating thicknesses to .ltoreq.14 .mu.m, however, restricts the use of the composite coatings in many erosive or abrasive wear applications, where longer life and resistance to large particle impact are crucial. It is therefore desirable to increase the thickness of the tungsten/tungsten carbide coating without inducing cracks to prolong the useful life of many components.
The disclosed composite coating system provides a means of increasing the thickness of the tungsten/tungsten carbide coating without inducing cracks. The composite coating system comprises a plurality of composite layers. Each of the composite layers comprises a first layer toward the substrate of tungsten and a second layer away from the substrate of a mixture of tungsten and tungsten carbide. The thicknesses of tungsten and tungsten/tungsten carbide in each composite layer are carefully controlled to provide crack free coating and to optimize the erosion and abrasion wear properties, load bearing capacity and resistance to large particle impact of the coating system.